1. Field of the Invention
The invention relates to a gear device that transmits power via gears.
2. Description of Related Art
A gear device transmits power through meshing engagement of gears. The gears rotate while constantly meshing or engaging with each other. Depending on the situation where the gear device is installed, there is an increasing demand for quietness. In the field of automobiles, for example, vibration noise generated in a vehicle as a whole has been reduced to some extent owing to various improvements. However, vibration noise caused by meshing engagement of gears is determined by the shape of the gears and the machining accuracy of the gears; therefore, the reduction of the noise has not progressed so much. Under the situation where surrounding vibration noise caused by other components is being reduced, the vibration noise caused by meshing engagement of the gears is becoming gradually noticeable.
Japanese Patent Application Publication No. 2008-275060 (JP 2008-275060 A) discloses a gear device for use in a railroad vehicle. In the gear device as described in JP 2008-275060 A, a tooth surface is subjected to 5 to 20 μm crowning in a direction of a contact line of the tooth surface, and gear dimensions are adjusted so that the contact ratio falls within the range of 2.4 to 2.8, while the tooth surface is modified so that the ratio of a contact length of a tooth contact face to a diagonal length of a plane of action falls within the range of 0.6 to 0.95.